Formation of polarized spectral lines in atmospheres with horizontal inhomogeneities

AbstractWe study the problem of the generation and transfer of spectral line intensity and polarization in models of stellar atmospheres with horizontal plasma inhomogeneities. We solve the non-LTE radiative transfer problem in full 3D geometry taking into account resonant scattering polarization and its modification by magnetic fields via the Hanle effect. We show that horizontal fluctuations of the thermodynamical conditions of stellar atmospheres can have a significant impact on the linear polarization of the emergent spectral line radiation and its center-to-limb variation.

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Discontinuities in numerical radiative transfer

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833984 ◽

2019 ◽

Vol 622 ◽

pp. A162 ◽

Cited By ~ 1

Author(s):

Gioele Janett

Keyword(s):

Radiative Transfer ◽

Transfer Equation ◽

Radiative Transfer Equation ◽

Transfer Problem ◽

Stellar Atmospheres ◽

Physical Parameters ◽

Radiative Transfer Problem ◽

Intermittent Behavior ◽

High Order Convergence ◽

Magnetohydrodynamic Simulations

Observations and magnetohydrodynamic simulations of solar and stellar atmospheres reveal an intermittent behavior or steep gradients in physical parameters, such as magnetic field, temperature, and bulk velocities. The numerical solution of the stationary radiative transfer equation is particularly challenging in such situations, because standard numerical methods may perform very inefficiently in the absence of local smoothness. However, a rigorous investigation of the numerical treatment of the radiative transfer equation in discontinuous media is still lacking. The aim of this work is to expose the limitations of standard convergence analyses for this problem and to identify the relevant issues. Moreover, specific numerical tests are performed. These show that discontinuities in the atmospheric physical parameters effectively induce first-order discontinuities in the radiative transfer equation, reducing the accuracy of the solution and thwarting high-order convergence. In addition, a survey of the existing numerical schemes for discontinuous ordinary differential systems and interpolation techniques for discontinuous discrete data is given, evaluating their applicability to the radiative transfer problem.

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Tomography of cool giant and supergiant star atmospheres

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731530 ◽

2018 ◽

Vol 610 ◽

pp. A29 ◽

Cited By ~ 3

Author(s):

K. Kravchenko ◽

S. Van Eck ◽

A. Chiavassa ◽

A. Jorissen ◽

B. Freytag ◽

...

Keyword(s):

Spectral Line ◽

Three Dimensional ◽

Velocity Fields ◽

Stellar Atmospheres ◽

Spectral Lines ◽

Line Of Sight ◽

Line Formation ◽

Tomographic Method ◽

1D Model ◽

Supergiant Star

Context. Cool giant and supergiant star atmospheres are characterized by complex velocity fields originating from convection and pulsation processes which are not fully understood yet. The velocity fields impact the formation of spectral lines, which thus contain information on the dynamics of stellar atmospheres. Aim. The tomographic method allows to recover the distribution of the component of the velocity field projected on the line of sight at different optical depths in the stellar atmosphere. The computation of the contribution function to the line depression aims at correctly identifying the depth of formation of spectral lines in order to construct numerical masks probing spectral lines forming at different optical depths. Methods. The tomographic method is applied to one-dimensional (1D) model atmospheres and to a realistic three-dimensional (3D) radiative hydrodynamics simulation performed with CO5BOLD in order to compare their spectral line formation depths and velocity fields. Results. In 1D model atmospheres, each spectral line forms in a restricted range of optical depths. On the other hand, in 3D simulations, the line formation depths are spread in the atmosphere mainly because of temperature and density inhomogeneities. Comparison of cross-correlation function profiles obtained from 3D synthetic spectra with velocities from the 3D simulation shows that the tomographic method correctly recovers the distribution of the velocity component projected on the line of sight in the atmosphere.

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Stark Broadening of Co II Lines in Stellar Atmospheres

Data ◽

10.3390/data5030074 ◽

2020 ◽

Vol 5 (3) ◽

pp. 74

Author(s):

Zlatko Majlinger ◽

Milan S. Dimitrijević ◽

Vladimir A. Srećković

Keyword(s):

Spectral Line ◽

Effective Temperature ◽

White Dwarf ◽

Semiempirical Method ◽

Stellar Atmospheres ◽

Spectral Lines ◽

Surface Gravity ◽

Type Star ◽

Half Maximum ◽

Stark Broadening

Data for Stark full widths at half maximum for 46 Co II multiplets were calculated using a modified semiempirical method. In order to show the applicability and usefulness of this set of data for research into white dwarf and A type star atmospheres, the obtained results were used to investigate the significance of the Stark broadening mechanism for Co II lines in the atmospheres of these objects. We examined the influence of surface gravity (log g), effective temperature and the wavelength of the spectral line on the importance of the inclusion of Stark broadening contribution in the profiles of the considered Co II spectral lines, for plasma conditions in atmospheric layers corresponding to different optical depths.

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The relative intensities of nitrogen lines in the semi-coronal regime

Journal of Plasma Physics ◽

10.1017/s0022377800007340 ◽

1973 ◽

Vol 9 (1) ◽

pp. 77-88 ◽

Cited By ~ 8

Author(s):

W. A. Cilliers ◽

J. D. Hey ◽

J. P. S. Rash

Keyword(s):

Spectral Line ◽

Electron Temperature ◽

Electron Density ◽

Line Intensity ◽

Intensity Ratio ◽

Spectral Lines ◽

Spectral Line Intensity ◽

Line Intensity Ratio ◽

Ratio Data ◽

Intensity Ratios

At plasma densities lower than about 1018 cm-3, strictly speaking, the LTE model cannot be used when the electron temperature is calculated from measured spectral line intensity ratios. In this work a number of nitrogen spectral lines were studied at an electron density of about 1016 cm-3, and the results are discussed with reference to the LTE and semi-coronal models. A method is suggested for analyzing line intensity ratio data from plasmas in the semi-coronal regime.

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From the Vector to Scalar Perturbations Addition in the Stark Broadening Theory of Spectral Lines

Universe ◽

10.3390/universe7060176 ◽

2021 ◽

Vol 7 (6) ◽

pp. 176

Author(s):

Valery Astapenko ◽

Andrei Letunov ◽

Valery Lisitsa

Keyword(s):

Spectral Line ◽

Stark Effect ◽

Coulomb Field ◽

Spectral Lines ◽

Alternative Methods ◽

Scalar Perturbation ◽

Numerical Comparison ◽

Line Shapes ◽

Stark Effects ◽

The Impact

The effect of plasma Coulomb microfied dynamics on spectral line shapes is under consideration. The analytical solution of the problem is unachievable with famous Chandrasekhar–Von-Neumann results up to the present time. The alternative methods are connected with modeling of a real ion Coulomb field dynamics by approximate models. One of the most accurate theories of ions dynamics effect on line shapes in plasmas is the Frequency Fluctuation Model (FFM) tested by the comparison with plasma microfield numerical simulations. The goal of the present paper is to make a detailed comparison of the FFM results with analytical ones for the linear and quadratic Stark effects in different limiting cases. The main problem is connected with perturbation additions laws known to be vector for small particle velocities (static line shapes) and scalar for large velocities (the impact limit). The general solutions for line shapes known in the frame of scalar perturbation additions are used to test the FFM procedure. The difference between “scalar” and “vector” models is demonstrated both for linear and quadratic Stark effects. It is shown that correct transition from static to impact limits for linear Stark-effect needs in account of the dependence of electric field jumping frequency in FFM on the field strengths. However, the constant jumping frequency is quite satisfactory for description of the quadratic Stark-effect. The detailed numerical comparison for spectral line shapes in the frame of both scalar and vector perturbation additions with and without jumping frequency field dependence for the linear and quadratic Stark effects is presented.

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Broadband Linear Polarization in Ap Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100020662 ◽

1993 ◽

Vol 138 ◽

pp. 305-309

Author(s):

Marco Landolfi ◽

Egidio Landi Degl’Innocenti ◽

Maurizio Landi Degl’Innocenti ◽

Jean-Louis Leroy ◽

Stefano Bagnulo

Keyword(s):

Magnetic Field ◽

Circular Polarization ◽

Linear Polarization ◽

Rotation Axis ◽

Spectral Lines ◽

Line Of Sight ◽

Long Time ◽

Rotating Star ◽

Ap Stars ◽

The Magnetic Field

AbstractBroadband linear polarization in the spectra of Ap stars is believed to be due to differential saturation between σ and π Zeeman components in spectral lines. This mechanism has been known for a long time to be the main agent of a similar phenomenon observed in sunspots. Since this phenomenon has been carefully calibrated in the solar case, it can be confidently used to deduce the magnetic field of Ap stars.Given the magnetic configuration of a rotating star, it is possible to deduce the broadband polarization at any phase. Calculations performed for the oblique dipole model show that the resulting polarization diagrams are very sensitive to the values of i (the angle between the rotation axis and the line of sight) and β (the angle between the rotation and magnetic axes). The dependence on i and β is such that the four-fold ambiguity typical of the circular polarization observations ((i,β), (β,i), (π-i,π-β), (π-β,π-i)) can be removed.

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The vaporized amounts, vaporization behavior and spectral line intensity of gold in graphite powder by arc discharge.

BUNSEKI KAGAKU ◽

10.2116/bunsekikagaku.17.1108 ◽

1968 ◽

Vol 17 (9) ◽

pp. 1108-1115

Author(s):

Hisao OSAWA

Keyword(s):

Spectral Line ◽

Line Intensity ◽

Arc Discharge ◽

Graphite Powder ◽

Spectral Line Intensity

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The relationship between photometric and spectroscopic oscillation amplitudes from 3D stellar atmosphere simulations

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab337 ◽

2021 ◽

Author(s):

Yixiao Zhou ◽

Thomas Nordlander ◽

Luca Casagrande ◽

Meridith Joyce ◽

Yaguang Li ◽

...

Keyword(s):

Radial Velocity ◽

Three Dimensional ◽

Quantitative Relationship ◽

Stellar Atmospheres ◽

Spectral Lines ◽

Wavelength Shift ◽

The Sun ◽

Theoretical Derivation ◽

Velocity Amplitude ◽

Good Agreement

Abstract We establish a quantitative relationship between photometric and spectroscopic detections of solar-like oscillations using ab initio, three-dimensional (3D), hydrodynamical numerical simulations of stellar atmospheres. We present a theoretical derivation as proof of concept for our method. We perform realistic spectral line formation calculations to quantify the ratio between luminosity and radial velocity amplitude for two case studies: the Sun and the red giant ε Tau. Luminosity amplitudes are computed based on the bolometric flux predicted by 3D simulations with granulation background modelled the same way as asteroseismic observations. Radial velocity amplitudes are determined from the wavelength shift of synthesized spectral lines with methods closely resembling those used in BiSON and SONG observations. Consequently, the theoretical luminosity to radial velocity amplitude ratios are directly comparable with corresponding observations. For the Sun, we predict theoretical ratios of 21.0 and 23.7 ppm/[m s−1] from BiSON and SONG respectively, in good agreement with observations 19.1 and 21.6 ppm/[m s−1]. For ε Tau, we predict K2 and SONG ratios of 48.4 ppm/[m s−1], again in good agreement with observations 42.2 ppm/[m s−1], and much improved over the result from conventional empirical scaling relations which gives 23.2 ppm/[m s−1]. This study thus opens the path towards a quantitative understanding of solar-like oscillations, via detailed modelling of 3D stellar atmospheres.

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